1. Field of the Invention
The present invention relates to an X-ray examination method and apparatus which are used in the fields of medical diagnosis and non-destructive examination.
2. Description of the Related Art
Conventionally, an X-ray examination apparatus such as an X-ray diagnostic apparatus or X-ray CT apparatus applies an X-ray beam emitted from an X-ray tube to an object and detects a transmitted X-ray image transmitted through the object by using a detector or the like, thereby visualizing an X-ray absorption image as a transmitted X-ray image which reflects the X-ray absorbing power difference between different X-ray propagation media contained in the object, as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 1-161645 (pp. 2-3).
Recently, attention has been paid to a new imaging principle of obtaining an image component of an X-ray refraction image which contour-enhances a boundary portion between different X-ray propagation media in superimposition on a general X-ray absorption image based on an X-ray absorbing power difference by using a technique of reducing the focal spot formed by an X-ray tube in combination with a technique of performing enlargement radiography by increasing the distance between an object and a detector as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-194738 (pp. 2-3, FIGS. 1 and 2). According to this imaging principle, an X-ray refraction image is formed in a region along the contour of the boundary surface between different X-ray propagation media in an object by the refraction of an X-ray beam by the boundary surface owing to the refractive index difference between the different X-ray propagation media. This imaging principle exhibits an improving effect of obtaining a clearer image and allowing identification of finer substances than in the prior art.
In the field of soft tissue medical diagnosis using mammography and the like, a screen-film system, storage fluorescent substance, or solid-state imaging device is generally used as a detector, and a low-energy X-ray beam is used by setting the tube voltage of an X-ray tube to as low as 20 to 39 kVp. In particular, a low-energy X-ray beam containing a characteristic X-ray of molybdenum (MO) or rhodium (RH) as a main component is generally used by using an X-ray tube using molybdenum (MO) or rhodium (RH) as an anode target. Using a low-energy X-ray beam makes it possible to obtain an identifiable absorption contrast even with respect to an abnormality which exhibits a relatively small X-ray absorbing power difference from neighboring normal tissue, e.g., an anomalous tissue in the breast.
In the field of non-destructive examination, when X-ray examination is to be performed on a foreign substance or a void (a cavity filled with air) inside a thick object, since it is necessary to ensure a transmitted X-ray dose with a sufficient intensity, the tube voltage of an X-ray tube is increased to increase the effective energy of incident X-rays as compared with a case of a thin object.